The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO (NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
The delivery of the AUS-32 solution to the exhaust involves precise metering of the fluid and proper preparation of the fluid to facilitate the later mixing of the ammonia in the exhaust stream. The precise metering is typically provided by the use of a low-cost, high volume solenoid injector commonly used in gasoline fuel systems.
The injector is supplied with AUS-32 via tubing that must be heated due to the relatively high freezing point of −11 C of the fluid. With reference to FIG. 1, the tubing interface 10 to the RDU 12 is generally a fluid connection that permits rapid coupling or decoupling of heated tubing (not shown) from the interface 10 for ease of assembly or for servicing. The RDU 12 is shown coupled to an exhaust flow path 13. The injector side of the connection is then welded or joined to the injector cup 14, into which is placed the injector inlet connector with O-ring 15 providing sealing. This connection results in an unheated volume (indicated by dimension A in FIG. 2) of fluid that is difficult to reach by the electrical heating element emanating from the tubing, thus increasing thaw times to thaw this volume.
Robert Bosch and Purem each have SCR systems in limited volume production for the heavy-duty diesel engine sector.
In the case of the Purem system, the metering control is carried out by a Siemens Deka IV injector mounted in a control block. The metered fluid is transported via a tube to the exhaust. After the metering valve, the fluid is also exposed to compressed air to aid with atomization which will ensure subsequent good mixing with the exhaust gas. The pressurized mixture is then injected into the exhaust. There is no provision for heating the fluid in the dosing unit. However, tubes supplying the dosing unit are heated and the system is purged with air at shutdown minimizing the amount of fluid that freezes. The fluid connection is made with a standard SAE J2044 quick connect fitting, creating the thawing problem noted above.
The Bosch production air-assist system also comprises a dosing module with heated supply tubes. Bosch has also disclosed a system concept that does not use air. Since air compression is not expected to be available on many future applications of the SCR technology, there is a need to have delivery of the AUS-32 without air-assistance while using a solenoid injector. In the conventional Bosch system without air assistance, the supply tubes to the injection unit are heated, with the injection unit interface to the fluid supply tubes employing the SAE J2044 quick connect fittings, creating the thawing problem noted above.
Thus, there is also a need to provide an improved fluid connection for an RDU that minimizes the volume of unheated fluid at the supply system interface.